You misunderstood me. I know how to uninstall drivers and how to change them in device-manager manually and tried all the aforementioned methods. Anyway thank you for the hints.
It just turned out that from one point on I couldn't switch back to MS drivers without having both IDE-channels shown with an exclamation mark (driver not meant for this system like error). Actually I just left it alone then, cause my Nvidia drivers worked without flaws and have a slightly better performance with my Hitachi HD.
Now that I investigated deeper into the source of the error I think the turning point was installing SP2. I found some references in the web that described the same error under different conditions, namely when using flash-cardreaders.
In the end it turns out to be a problem of "atapi.sys", which I managed to solve now. So my MS drivers are up and running smoothly again.
If that has any positive impact beside being able to use the Liteon utilities will have to be experienced by time. One thing I can say is that with Nvidia drivers I get a strange error message in my system log when I put my Plextor 48/24/48a as slave on primary channel with my Hitachi HD. It only happens with that configuration and actually has no impact on the system. Nevertheless it disappears with MS drivers.
I just checked the ASPI driver details under the device-manager, it's an Adaptec version 4.57, so I am going to look for an update. Actually I thought with the installation of Nero it would be exchanged by Nero's.